Radiation treatment has been used for treating patients with cancer for almost 100 years.
A. Problems with Radiation Therapy
Today, radiation treatment is commonly used as a form of therapy for treating primary and metastatic tumors. Typically, a radiation beam is focused at a finite number of known or suspected tumors with the radiation doses and path of radiation designed to minimize damage to surrounding, non-target, normal cells. Obviously, for beam focused radiation therapy to be successful, the location of a finite number of tumors must be known. For example, beam focused radiation for the treatment of primary or metastatic cancer of the liver is hampered by diffuse infiltration of the liver by secondary or primary liver cancer. In addition, the liver is a tissue that is sensitive to radiation, see below.
A second type of radiation therapy involves non-selective whole body treatment with high doses of irradiation. In this type of therapy, bone marrow is removed prior to irradiation to avoid irradiation of the stem cells. (The bone marrow stem cells give rise to the cells of immune and blood systems). The patient then undergoes whole body irradiation, which kills both cancer cells, whose location may or may not be known, and also kills cells of the blood and immune system. While the patient is being irradiated, the bone marrow is purged of contaminating cancer cells, by one of a variety of techniques. The purged marrow is replaced into the patient after radiation has been completed. The purging of marrow and replacement in the patient is sometimes called autologous bone marrow transplantation. In autologous bone marrow transplantation, radiation is used non-selectively, on the whole body, the selective non-irradiation of the stem cells being accomplished by physical removal of those cells from the patient. Autologous bone marrow transplantation is expensive and risky because the patient lacks an immune system until the replaced bone marrow begins to proliferate and yields functioning blood and immune cells. Currently, autologous bone marrow transplantation is used only in hematological cancers that have not responded to standard forms of chemotherapy. Thus, radiation therapy is either tumor specific by virtue of the path of the radiation beam(s), or selective by virtue of the removal of stem cells.
B. Chemistry of Radiation Damage
Radiation disrupts cells by fatally altering the genetic and somatic functions of cellular constituents. DNA is particularly sensitive to radiation. Damage to DNA occurs either by direct radiation or by reaction with hydroxyl radicals produced from radiolysis of neighboring water molecules. Reaction with oxygen produces peroxyl radical DNA-O.sub.2 which forms products that cause irreversible damage to the DNA unless counteracted with reducing species such as a thiol to restore the original molecule (Coleman, in: Cancer, Principles and Practice of Oncology, 3rd Ed., DeVita et al., Eds., Phila.: JB Lippincott Co., 1989). Consequently thiols have an established radioprotective activity. Because of the central role oxygen is believed to play in radiation damage, the cells most vulnerable to radiation are those that are well oxygenated and are dividing. These include stem cells (bone marrow cells) and cells that are highly perfused with oxygen, for example, liver cells, endothelial cells and epithelial cells in the lung. In some cases the sensitivity of normal cells to relatively low radiation doses is great, and the adverse effects caused by radiation of non-target cells may offset any advantage accrued from reducing the tumor load.
C. Sensitivity of the Liver to Radiation
The treatment of liver cancer with radiation both embodies many of the problems of radiation therapy and is a specific embodiment of the current invention.
At the time of detection of primary or secondary cancer within the liver, there are often multiple tumors within the liver that prevent surgical resection. In the extreme case, the cancer can be diffuse, or exist not as discrete tumors, but intermixed with normal tissue throughout the liver. Radiation therapy is hampered by the liver's inherent sensitivity to radiation.
In the liver approximately 5,000 rads is considered necessary to achieve a high probability of control over a subclinical adenocarcinoma (i.e., &lt;10.sup.6 cells), and 6,000 rads to control a squamous cell tumor of less than 2 cm (Hall et al. in: Radiation and Oncology Rationale, Moss et al. Eds., 1989, pp. 1-57). However, the liver has a low radiation tolerance (2500-3000 rad), and radiation doses of more than 3,000 rad within 3 weeks can result in radiation hepatitis (Wanebo et al. In: Cancer Principles and Practice of Oncology. Hellman et al. Eds., Phila.: JB Lippincott, 1989, pp. 836-874). To treat liver tumors successfully with radiation there is a need to develop a method of treatment that spares the hepatocytes while increasing the damage to tumors (Ono et al. in: Radiation Research, 1990, 123:345-347.
D. Pharmaceutical Agents Used to Alter Effects of Radiation
Attempts to use pharmaceutical agents in conjunction with radiation for the treatment of cancer have often involved two general approaches: (1) lowering the radiation tolerance of the tumor, i.e., making the tumor more susceptible to radiation; and (2) raising the radiation tolerance of non-target cells by protecting them from or reducing the damage to them during radiation therapy. For the former method, compounds such as hypoxic cell sensitizers are administered on the theory that hypoxic cells, such as tumor cells, are less sensitive to radiation (Coleman, in: Cancer, Principles and Practice of Oncology, 3rd Ed., DeVita et al., Eds. Phila.: JB Lippincott Co., 1989, pg. 2436). The latter form of radioprotection attempts to eliminate or minimize the effects of radiation on non-target tissue. Radioprotecting agents can be classified as falling into two general types. They are biologically active radioprotective agents and chemoactive radioprotective agents.
1. Biologically Active Radioprotective Agents
Some agents are radioprotective because they possess a biological activity which, after radiation, ameliorates or compensates for the damage produced by radiation. Their indirect radioprotecting capability stems from the abilities of the agents to modify biological responses, such as to suppress or stimulate the immune systems, or initiate or accelerate repair of damaged tissue. Molecules that stimulate repair or replacement of damaged cells include certain hormones and hormone-like compounds, immuno-potentiators, immuno-stimulators and anti-inflammatory agents. Gohla (U.S. Pat. No. 5,096,708) disclosed that an active component derived from Thuja occidentalis that contains polysaccharides acts as an immune-stimulant assisting the reconstitution of bone marrow cells after radiation damage has occurred. Similarly, Wagner et al. (DE 3,042,491) report that infections which arise in a person after radiation damage can be effectively prevented or controlled by a polysaccharide derived from a plant of the compositae family that acts as an immune stimulant.
Another approach to protecting non-target cells from radiation is to inhibit their growth and differentiation. Keller et al. (U.S. patent application Ser. No. 7,372,815) used antibodies to transforming growth factor-beta-1 to inhibit hematopoietic progenitor growth and differentiation prior to radiation and thereby reduce susceptibility of these cells to radiation damage, an effect that is greatest in dividing cells.
2. Chemoactive Radioprotective Compounds
Chemoactive radioprotectants compounds (CRC's) are agents which directly protect cells from radiation damage. Examples of chemoactive radioprotective agents include low molecular weight compounds such as free radical scavengers or anti-oxidants (vitamin E, ascorbic acid). Sulfhydryl compounds (compounds containing an SH-group) and cationic thiols (sulfhydryl compounds containing an amine group), function as free radical scavengers, and accordingly have been evaluated for use as chemoactive radioprotectants. Examples of cationic thiols that act as protective agents are S-2-(3-aminopropylamino) ethyl phosphophorothioic acid (WR2721), a thiophosphate derivative of aminothiol cysteamine and 2-(3-aminopropyl) aminoethanethiol (WR1065). Some CRC's are high molecular weight molecules. These include biological molecules that have anti-oxidant properties such as the enzyme superoxide dismutase, which destroys the oxygen anion (superoxide) generated during radiation and reduces the concentrations of mediators of radiation damage. Other radioprotectant agents include selenium, melanins, cysteamine derivatives, phenolic functional groups (such as 6-hydroxy-chroman-2 carboxylic acids (e.g., Trolox.RTM.) and tocopherols (e.g., Vitamin E), and enzymes (superoxide dismutase). Table 1 lists selected CRC's.
Current CRC's suffer from the following limitations:
1. Low molecular weight CRC's are rapidly excreted by glomerular filtration of the blood through the kidney. This limits both the uptake of the CRC by normal tissue and the time the CRC is in the body.
2. The ability of CRC's that are macromolecular (high molecular weight) to protect against radiation damage is limited because macromolecules do not cross membranes surrounding cells. They do not enter the cytoplasm and are confined to a location far from the DNA molecules that need their protection.
3. Both high and low molecular weight CRC's lack cell or organ selectivity. For example, plasma distributes low molecular weight thiols throughout the extracellular fluid around cells as well as within cells in the intracellular fluid. To achieve concentrations sufficient for the radioprotection of any particular cell or tissue, high concentrations of CRCs must be achieved in all cells. This lack of selectivity increases the dose of agent that must be administered and increases the possibility of toxic reactions to the agent.
4. Both high and low molecular weight CRC's lack adequate selectivity for normal tissue over tumor tissue. Probably the best known CRC is S-2-(3-aminopropylamino) ethylphosphorothioic acid, a thiophosphate derivative of aminothiol cysteamine known as WR2721. Developed earlier as a protective agent for battlefield radiation, WR 2721 has been the subject of recent clinical investigations. The major obstacle to the use of WR2721 in radiotherapy of liver cancer is its non-selectivity, which resulted in the rejection of WR2721 as a protecting agent (The Pink Sheet, Feb 3, 54, #5 (1992)).